Today I get to pretend to be a big-time radio DJ, a regular Casey Kasem or Delilah*, by sending out this post by request to Mike Pope, one of this blog’s earliest followers. (If you’ve got a similar simmering question, drop a line at motivatedgrammar at gmail dot com and I’ll try to look into it.) He suggested I look into the debate between all of a sudden and all of the sudden, and it turned out to be a pretty interesting topic.
To state the situation briefly, all of a sudden is the undisputed champion in contemporary English. Whether you’re looking at Google Books N-grams, Google N-grams, the Corpus of Historical American English (COHA), or the Corpus of Contemporary American English (COCA), it’s a sudden by far. The gap between the two seems to widen as the formality and amount of editing increases. Google N-grams, composed of web data, has the smallest ratio between the two, while Google Books N-grams, composed of polished published works, has the largest:
Google: 12.5 a suddens per 1 the sudden
Google Books: ~50 a suddens per 1 the sudden
Part of this is that the sudden seems to be a recent option. It started blowing up on Google Books only since 1985, and doesn’t appear at all in COHA until the 90s. Furthermore, there isn’t any complaint about it in any of the grammar books on my bookshelf, so that suggests that it wasn’t much of a concern before 2000, when the most recent book I looked at was published. (Garner’s Modern American Usage mentions it in the 2003 edition.) But it is definitely a concern for Internet grammarians:
Urban Dictionary: “All of the sudden: A stupid variation for ‘all of a sudden’, which people who are stupid use.”
“All of the sudden” and “all the sudden” are not correct phrases. They are slang.
“All of a sudden” is the correct and only way to say it. “All of the sudden” is bad grammar and wrong. [...] What’s ‘the sudden’? And what’s ‘all of’ it anyway? Sudden is an adjective. So, I use All of a sudden.
Of course, that last argument makes no sense; if the sudden is an ungrammatical usage of an adjective as a noun, there’s no reason why a sudden would be acceptable. Not that it matters; all of a/the sudden is an idiom and thus grammaticality becomes of secondary concern. If an idiom is acceptable, then it’s acceptable, whether or not the grammar predicts it would be.
So let’s set aside the specific insults for a moment and look at why the sudden is thus maligned. Some claim that all of the sudden is regional, and place it in various low-prestige localities. In this discussion, for instance, various commenters localize it to either Cajun Louisiana, the general Southern U.S., the Midwestern U.S., or northern England, all of which have little in common except for grammaticasters’ disdain for their Englishes as lazy and uneducated.
But I don’t see much evidence of localization in contemporary (American) English. The first two celebrities I found using the sudden were Matt Lauer and Michael Douglas, both born in the New York/New Jersey area. To test the claim a bit more fairly, I compiled a quick map of the 100 most recent American examples of all of a sudden and all of the sudden from Twitter. The red dots represent tweets containing the phrase, with darker dots indicating more tweets from that town.** Before you look at the caption, can you tell which map is which phrase?
I see no evidence here of regionality. That’s not to say that the sudden was never regional at any point in its history, but I think that right now the sudden is merely informal and/or non-standard, rather than regionalized.
Speaking of history, the interesting thing is that, even as we now say that the sudden is the new form, it was also the earlier form. The Oxford English Dictionary shows the sudden as the earliest — and now archaic — form, appearing in examples such as:
I thinke, that none can iustly account them selues Architectes, of the suddeyne. 
I was‥compelled‥to answere of the sodaine vnto such articles. 
Of course, the history doesn’t matter now, but I do love when a variant that’s now considered improper used to be the standard. Maybe in a couple hundred years, the two options will have switched again.
Summary: All of a sudden is the standard idiom in contemporary English. All of the sudden is a newer non-standard variant that does not appear to be geographically localized. Interestingly, the sudden is the original form if you go back to the 1500s.
*: I actually was a radio DJ for 25 minutes late one spring night in 2002 on WPRB 103.3 Princeton. Unable to find enough familiar songs in the station’s archives, I was forced to play a track from Spokane’s soporific album Leisure and Other Songs alongside the pounding metallic God Inside My Head by the Catherine Wheel. It was a daring juxtaposition that I hope was appreciated by the three or so people listening to college radio at 2 in the morning.
**: By the way, I’m putting this utility together on a website that I’m intending to launch next Wednesday, so you can do similar comparisons yourself.